jemalloc assertion failures during rustc compilation

[RalfJung]

Properly enabling jemalloc assertion leads to rustc builds failing. This is not currently caught by our CI as the debug-jemalloc option in config.toml is broken. See #43648 for some more information on this, and https://travis-ci.org/rust-lang/rust/jobs/266493535 for a failing test log:

Building rustdoc for stage2 (x86_64-unknown-linux-gnu)
[00:50:11]    Compiling bitflags v0.8.2
[00:50:11]    Compiling pulldown-cmark v0.0.14
[00:50:11]    Compiling libc v0.2.29
[00:50:11]    Compiling log v0.3.8
[00:50:11]    Compiling gcc v0.3.51
[00:50:12] <jemalloc>: /checkout/src/liballoc_jemalloc/../jemalloc/src/jemalloc.c:2654: Failed assertion: "usize == isalloc(tsd_tsdn(tsd), ptr, config_prof)"
[00:50:12] error: Could not compile `libc`.
[00:50:12] warning: build failed, waiting for other jobs to finish...
[00:50:24] error: build failed